What is combustible dust?
Combustible dust is a material that is capable of igniting or burning when suspended in air. Combustible dust is usually characterized by particulate with an average diameter of 420 micrometers (μm) or smaller and a moisture content of less than 5 %.
How is combustible dust tested?
Combustible dust analysis consists of a series of tests both to determine particle size and moisture content and to determine the explosion severity of the dust. The sampling part is fairly simple — dust is carefully collected using a brush and put into a glass or plastic jar. It is suggested to collect samples from the appropriate areas including production locations (where machinery is located) and elevated surfaces. Depending on the analysis, 1-2 kilograms of dust must be collected and submitted to the lab for analysis. Air sampling is not necessary to determine whether or not a dust is combustible. In regards to sampling combustible dust, EMSL recommends following the OSHA ID-201SG sampling method guideline. The OSHA Combustible Dust Emphasis Program (CPL 03-00-008) also provides information on sample collection, definitions related to combustible dust, types of dust, description of the testing procedures, etc.
Combustible dust is typically analyzed by a suite of testing:
Initial Dust Characterization
Determines particle size and moisture content. The most important information determined in this stage is the Percent Combustible Dust. This is the percentage of the sample that has the potential to be combustible when it is dry and fine enough to pass through a 40 mesh sieve (less than 420 μm). The testing includes:
• Percent through 40 Mesh Screen
• Percent Moisture Content
• Percent Combustible Material (calculated)
• Percent Combustible Dust (calculated)
Go – No Go Testing (Explosive Screening) – ASTM E1226
This is an economical and practical way to determine if the dust in the sample has the potential to be explosive. Testing consists of exposing the fine dust in the sample to low energy igniters inside the 20-Liter Siwek chamber. Explosive dusts will generate a measurable overpressure. Non- explosive dusts are not considered a threat and the analysis can be aborted to avoid unnecessary fees. If the sample is explosive on the screening test, the more comprehend analyses listed below should be conducted.
Combustibility Screening Test - VDI 2263 Part 1
A dust is considered explosible if there is a flame propagation after igniting the dust/air mixture resulting in a pressure rise in a closed vessel. This test is conducted in the Hartmann 1.2 Liter Vessel. The sample is tested over a range of concentrations and is submitted to a continuous induction spark as an ignition source.
Explosion Severity (Kst, Pmax, [dP/dt]max) – ASTM E1226
This testing provides an indication of the severity of the dust explosion by determining the deflagration parameters. Larger Kst values represent a higher explosion severity. For this test, the dust is suspended and ignited in the Siwek chamber over a range of concentrations and the maximum pressure and the pressure rise with respect to time are measured.
Minimum Explosion Concentration (MEC) – ASTM E1515
MEC is the minimum concentration of a material to cause a measureable explosion overpressure. It is determined by suspending the dust in the Siwek Chamber.
Minimum Ignition Energy (MIE) – ASTM E2019
MIE is the electrical energy discharged from a capacitor, just sufficient to produce the ignition of the most ignitable mixture of air and dust. It is determined by suspending the dust in a Hartmann 1.2 Liter Vessel explosion tube.
Minimum Ignition Temperature Test (MIT) – ASTM E1491 (dust cloud) ASTM E2021 (dust layer)
This test method covers the minimum temperature at which a dust cloud will autoignite when exposed to heated air from furnace at atmospheric pressure. It is determined by introducing the dust into a BAM oven. As an alternative, the minimum temperature of self-ignition of a dust layer can be measured using a hot plate set-up (see Layer Ignition Temperature).
Layer Ignition Temperature (LIT) – ASTM E2021
Dust Layer - This test method covers the minimum temperature at which a dust cloud will auto ignite when exposed to air and heated in a furnace at atmospheric pressure. It is determined by introducing the dust into a BAM oven. As an alternative, the minimum temperature of self-ignition of dust layer can be measured using a hot plate set-up.
Class II Testing
This level of testing involves a number of parameters that determine if the sampled dust is considered a Class II hazardous material. Class II locations are defined as locations with combustible dust having Ignition Sensitivity (I.S.) greater than or equal to 0.2 or Explosion Severity (E.S.) greater than or equal to 0.5. I.S. is calculated from MIT, MIE, and MEC for the sample normalized to Pittsburg coal dust, whereas E.S. is calculated from Pmax and [dP/dt]max for the sample, also normalized to Pittsburg coal dust.
Resistivity Testing (for metal dust in particular) – ASTM D257
The resistivity testing is particularly important for metal dust. The electrical nature of the dust is one criteria to determine if it is necessary to take special precaution with regard to electrical insulation of the equipment operating in a location with Class II dust.
Limiting Oxygen Concentration (LOC) – ASTM E2931
This test method is designed to determine the limiting oxygen concentration of a combustible dust dispersed in a mixture of air with an inert/nonflammable gas. The testing is performed in the 20-L Siwek chamber.
Ignitability of Solids - EPA 1030/CFR § 49.173 –
This method is suitable for the determination of the ignitability of solids. Material is formed into an unbroken strip or powder train. An ignition source is applied to one end of the test material to determine whether combustion will propagate within a specified time period. If propagation occurs, material is submitted to a burning rate test. In the burning rate test, burning time is measured over a distance and a burning rate is determined. Materials that do not ignite or propagate combustion do not require further testing.